Image capture devices, such as digital cameras, typically include an optical image capture sensor that converts an image focused onto the sensor into an electrical signal, and an optical component having a lens for focusing the image onto sensor.
Optical image capture sensors of the type used in digital cameras are typically provided in the form of a monolithic electronic chip, known as an optical sensor die. The optical sensor die is typically mounted in a package that provides support and protection to the die, and includes electrical contacts, or leads, for connecting the die to an electrical circuit board. The package, with the die installed, is typically combined with optical components having lenses to focus light on the die to form an optical sensor module that is attached to a circuit board inside the camera.
The lenses in such optical sensor modules are typically formed from a thermoplastic material, such as an optical grade of polycarbonate having a low melting temperature, rather than glass, to minimize the cost of producing the lenses. These thermoplastic materials typically have a melting temperature of about 150°Celsius (C.). One type of polycarbonate material used for the lenses in optical sensor modules is sold by the General Electric Company under the trade name LEXAN®.
Although it would be desirable for minimizing the size and cost of the module, and for effectively and efficiently removing heat from the sensor die, to solder the module directly to the circuit board using a high-volume production process such as solder reflow, the thermoplastic materials used for forming the lenses cannot withstand the temperatures in the range of 220° to 260° C. that are required for melting the solder to join a module to a circuit board using such processes.
This inability of the lenses to withstand the temperature that would be generated in soldering the module directly to the circuit board has necessitated the use of intermediate coupling devices for electrically connecting and physically mounting the module on the circuit board.
In one commonly used mounting approach, a socket is soldered to the circuit board. The optical sensor module is inserted into the socket, subsequent to the soldering operation, to establish electrical contact and provide physical support of the module. This approach does not allow the module to be miniaturized due to the need for the socket, and connection features on the module that are required for engaging electrical contacts in the sockets and for retaining the module in the socket.
In another commonly used approach, a connector for a flex connection is soldered directly to the circuit board. A flex circuit or cable is then used for electrically connecting the module to the connector on the circuit board. The module is then mechanically attached and supported on the circuit board by a mounting device. This approach also does not allow for the module to be miniaturized, due to the need for the connector, the flex connection, and the module mounting device.
Using a socket or a flex connection for mounting the module on the circuit board are also undesirable because the type of packages typically used in these approaches, for housing optical image capture sensor dies, do not provide for efficient and effective removal of heat generated in the optical sensor die.
It would be desirable to utilize a relatively new type of electronic package technology, known as a Leadless Leadframe Package (LLP) for packaging an optical image capture sensor of the type used in digital cameras. In an LLP, a metal lead frame substrate is formed by a process such as chemical etching from a thin sheet of metal (typically copper). The LLP includes at least one cut-out extending between parallel internal an external surfaces of the lead frame, with the cut-out forming a die-attach pad and one or more leadless connection pads. A die is mounted on the die-attach surface, and wire bonds or other types of electrical connections are made between the die and the internal surfaces of the connection pads. An electrically insulating encapsulating material is then molded around the lead frame, die and wire bonds. After the encapsulation is cured, the LLP is trimmed to final shape. U.S. Pat. No. 6,143,981, to Glenn, and U.S. Pat. No. 6,372,539 B1, to Bayan, et al, disclose LLP electronic packages of the type described above.
An LLP differs from other types of lead frame based packages in that there are no metal leads extending from the finished LLP for insertion into or attachment to pads on a printed wiring board (PWB). In an LLP, the connection pads are used for forming a leadless connection by applying a ball of solder crème or paste between the connection pads of the LLP and mating pads on the PWB, and subjecting the PWB to a process such as solder reflow. U.S. Pat. No. 4,927,697, to IHill, describes a method for attaching and LLP to a PWB.
Prior LLP packages are not amenable, however, to packaging an optical image capture sensor of the type used in digital cameras. The performance of an image capture sensors is so seriously degraded by even minute particles of dust, interfering with an image reaching the surface of the sensor, that electronic packages containing optical image capture sensors must be assembled in closely controlled clean room environments. The structure and encapsulation typically used in prior LLP modules would interfere with the image capture sensor receiving the image in sufficient detail.
What is needed is an improved apparatus and method for mounting an optical image capture sensor die on a printed wiring board, preferably through the use of an LLP-type package.